OBJECTIVES: 1) Completion of study of natural degeneration in brachial and lumber spinal ganglia of chick embryo. Determination of duration and peak periods of degeneration for the large, early differentiating and the small late differentiating neurons. 2) Completion of the study of effects of systematically injected Nerve Growth Factor (NGF) on natural degeneration of spinal neurons. It was established that cell death in both the large and small neuron populations is significantly reduced by NGF. This is the first evidence that - contrary to previous contentions - the large cells are susceptible to NGF. 3) Efforts by neuron counts at stages preceding neuronal death in the population of small cells to decide the controversial questions of whether or not NGF can regulate proliferation of pre-neuroblasts.